test: change concatenated string to template

PR-URL: #16929
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Anatoli Papirovski <apapirovski@mac.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Franziska Hinkelmann <franziska.hinkelmann@gmail.com>

test/fixtures/net-fd-passing-receiver.js

@@ -15,12 +15,12 @@ receiver = net.createServer(function(socket) {
     });
 
     passedSocket.on('data', function(data) {
-      passedSocket.send('[echo] ' + data);
+      passedSocket.send(`[echo] ${data}`);
     });
     passedSocket.on('close', function() {
       receiver.close();
     });
-    passedSocket.send('[greeting] ' + greeting);
+    passedSocket.send(`[greeting] ${greeting}`);
   });
 });